A programmable logic switch is a device for controlling ON/OFF states of a logic switch (for example, transistor and the like) in accordance with data held in a memory. In general, the programmable logic switch is used for a programmable logic device (For example, an FPGA (Field Programmable Gate Array) and the like) that requires reconfiguration of a logic operational circuit and an interconnection circuit.
A programmable logic switch used in a conventional FPGA uses a volatile memory such as an SRAM. For this reason, data saved in the memory is lost when the power is turned off. Therefore, when the power is turned on again, there is a problem in that it is necessary to read the data from a separately provided memory region again. In general, the SRAM includes six transistors. Therefore, an FPGA using many SRAMs has a problem in that the chip size increases.
For this problem, a programmable logic switch using a nonvolatile flash memory is known. For example, a memory cell of the FPGA includes a first nonvolatile memory device and a second nonvolatile memory device which are connected in series. A contact point between a first nonvolatile memory device and a second nonvolatile memory device serves as an output node Q of the memory cell. The output node Q is connected to an NMOS transistor and a switching transistor.
When an integrated circuit having both the nonvolatile memory device and the switching transistor in a mixed manner is achieved, the manufacturing process of the switching transistor and the manufacturing process of the nonvolatile memory device should be preferably similar to each other. This is because, if the nonvolatile memory device and the switching transistor can be made with a similar process, both of them can be arranged in proximity to each other within the chip, and the size of area can be reduced which reduces the lengths of wires (conductive lines), and moreover, this improves the operation speed.
However, different performances and voltage conditions are required of the nonvolatile memory device and the switching transistor, and it is not desirable for both of them to have the same impurity concentration in the semiconductor, gate length, and the like. In order to achieve a high-performance nonvolatile programmable switch at a low cost, manufacturing processes (conditions) of the nonvolatile memory device and that of the switching transistor are respectively optimized, and further, they also need to be similar to each other. A nonvolatile programmable switch for achieving such high-performance and low cost has not been sufficiently studied.